1. Field of the Invention
The present invention relates generally to a polyolefin reactor system having a gas phase reactor and, more particularly, to a reactor system employing a motive device to facilitate recovery of polymer solids from overhead gas of the gas phase reactor.
2. Description of the Related Art
This section is intended to introduce the reader to aspects of art that may be related to aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior an.
As chemical and petrochemical technologies have advanced, the products of these technologies have become increasingly prevalent in society. In particular, as techniques for bonding simple molecular building blocks into longer chains (or polymers) have advanced, the polymer products, typically in the form of various plastics, have been increasingly incorporated into everyday items. Polyolefin polymers such as polyethylene, polypropylene, and their copolymers, are used for various films, piping, retail and pharmaceutical packaging, food and beverage packaging, plastic bags, toys, carpeting, various industrial products, automobile components, appliances and other household items, and so forth.
Specific types of polyolefins, such as high-density polyethylene (HDPE), have particular applications in the manufacture of blow-molded and injection-molded goods, such as food and beverage containers, film, and plastic pipe. Other types of polyolefins, such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), isotactic polypropylene (iPP), and syndiotactic polypropylene (sPP) are also suited for similar applications. The mechanical requirements of the application, such as tensile strength and density, and/or the chemical requirements such as thermal stability, molecular weight, and chemical reactivity, typically determine what type of polyolefin is suitable.
One benefit of polyolefin construction, as may be deduced from the list of uses above, is that it is generally non-reactive with goods or products with which it is in contact. This allows polyolefin products to be used in residential, commercial, and industrial contexts, including food and beverage storage and transportation, consumer electronics, agriculture, shipping, and vehicular construction. The wide variety of residential, commercial and industrial uses for polyolefins has translated into a substantial demand for raw polyolefin which can be extruded, injected, blown or otherwise formed into a final consumable product or component.
To satisfy this demand, various processes exist by which olefins may be polymerized to form polyolefins. These processes may be performed at or near petrochemical facilities, which provide ready access to the short-chain olefin molecules (monomers and comonomers), such as ethylene, propylene, butene, pentene, hexene, octene, decene, and other building blocks of the much longer polyolefin polymers. These monomers and comonomers may be polymerized in a liquid-phase polymerization reactor and/or gas-phase polymerization reactor. As polymer chains develop during polymerization in the reactor, solid particles known as “fluff” or “flake” or “powder” are produced in the reactor.
The fluff may possess one or more melt, physical, rheological, and/or mechanical properties of interest, such as density, melt index (MI), melt flow rate (MFR), comonomer content, molecular weight, crystallinity, and so on. Different properties for the fluff may be desirable depending on the application to which the polyolefin fluff or subsequently pelletized polyolefin is to be applied. Selection and control of the reaction conditions within the reactor, such as temperature, pressure, chemical concentrations, polymer production rate, catalyst type, and so forth, may affect the fluff properties.
In addition to the one or more olefin monomers, a catalyst (e.g., Ziegler-Natta, metallocene, chromium-based, post-metallocene, nickel, etc.) for facilitating the polymerization of the monomers may be added to the reactor. For example, the catalyst may be a particle added via a reactor feed stream or recycle stream and, once added, suspended in the fluid medium within the reactor. Unlike the monomers, catalysts are generally not consumed in the polymerization reaction. Further, as appreciated by the skilled artisan, the catalyst particle morphology may be supported or unsupported.
The polymerization may be performed in a single reactor or in multiple reactors in series and/or parallel. For example, one or more loop slurry (liquid phase) reactors, one or more gas phase (e.g., fluidized bed) reactors, or combinations of loop slurry reactors and gas phase may be employed. The product discharge of the reactor or terminal reactor generally has the desired polymer polyolefin fluff and may be further processed to deactivate residual catalyst and remove non-polymer components. The polyolefin fluff may be sent to the customer in a non-pelletized form, or pelletized in an extruder and sent to the customer in pellet form.
In the case of a fluidized-bed gas phase reactor to polymerize olefin into polyolefin, the carryover of polyolefin solids (typically primarily polymer fines) in the reactor overhead system can be problematic. Indeed, the presence of polymer fines over time can result in fouling or plugging of the reactor overhead system (including the solids recovery system having a separation vessel and motive device), causing an unplanned shutdown of the reactor system, resulting in loss polyolefin production and increased maintenance costs.